He invented the use of percentile grades for showing distribution of phenomenon. The height charts that you saw as a kid (at age three 25% of children are above x inches, 50% of children are above y inches, etc.) are a Galtonian invention. This method was a simple but useful way of showing normal distributions.
While Galton’s work in measurement and data visualization was groundbreaking, it also represents the beginning of the Eugenics Movement. Galton himself coined the term after his studies indicated that many desirable traits were hereditary in nature.
For a complete treatment of the history of statistics, see History of Statistics.
Galton is also known for Galton’s Distribution, more commonly called the Log-Normal Distribution.
Galton’s Anthropometric Laboratory at The 1884 International Health Exhibition was an early intersection of data collection and entertainment. [http://www.galton.org/galton/essays/1880-1889/galton-1884-anthro-lab.pdf pdf]